Since the late 1970s, photonic devices have increasingly supplanted conventional electronic devices for transmitting data. Rather than encoding data in electrical signals and transmitting the encoded electrical signals via signal lines, the data can be encoded in electromagnetic waves and transmitted via photonic devices, such as optical fibers and photonic crystal waveguides. Transmitting encoded electromagnetic waves via photonic devices have a number of advantages over transmitting encoded electrical signals via signal lines. First, signal degradation or loss is much less for electromagnetic waves transmitted via photonic devices than for electrical signals transmitted via signal lines. Second, photonic devices provide a much higher bandwidth than signal lines. For example, a single Cu or Al wire can only transmit a single electrical signal, while a single optical fiber can be configured to transmit about 100 or more electromagnetic waves. Finally, electromagnetic waves provide a much higher data transmission rate and eliminate electromagnetic interference.
Recently, advances in materials science and semiconductor fabrication techniques have made it possible to fabricate computational devices that integrate photonic devices with electronic devices, such as memory and processors. In particular, photonic integrated circuits (“PICs”) are the photonic device equivalent of electronic integrated circuits. PICs can be implemented on a small wafer of semiconductor material that forms the base for an integrated circuit and may include a number of waveguides for transmitting data encoded in electromagnetic waves to a number of integrated photonic and electronic devices. Unlike electronic integrated circuits where Si is the primary material, PICs may be composed of a variety of materials. For example, PICs may be composed of a single semiconductor, such as Si on an insulator, or binary and ternary semiconductors, such as InP and AlxGa1-xAs, where x varies from 0 to 1.
In order to effectively implement PICs, a number of passive and active photonic components are needed. Waveguides and attenuators are examples of passive photonic components that can be fabricated using conventional epitaxial and lithographic methods and may be used to direct the propagation of electromagnetic waves between electronic devices. Physicists, engineers, and computer scientists have recognized a need for active photonic components, such as modulators and switches, which can be implemented in PICs to encode data in, and regulate transmission of, electromagnetic waves.